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Pecan Scab

Severe scab infections on nuts. These nuts will drop prematurely or become sticktightsUniversity of Missouri

Early scab infections on underside of leafUniversity of Missouri

Scientific Name: Cladosporium caryigenum

Life cycle

The pecan scab fungus overwinters as a small, tight mat of fungal material called a "stroma" on shucks, leaf petioles and stems infected the previous season. With warmer temperatures and rainfall in the spring, fungal spores are produced on the stroma. Dew and rain spread spores locally within a tree, and the wind spreads them over long distances to adjacent trees or orchards.

Description

Pecan scab first appears as small, circular, olive-green spots that turn to black on the newly expanding leaves, leaf petioles and nut shuck tissue (see Figures 5 and 6). All tissues are most susceptible when young and actively growing. Lesions expand and may coalesce. Old lesions crack and fall out of the leaf blade, giving a shot-hole appearance. Nut infections cause the greatest economic damage. Early infections may cause premature nut drop but more commonly cause the shuck to adhere to the nut surface, causing sticktights. Late infections can prevent nuts from fully expanding and decrease nut size.

Control

Resistant varieties offer the first line of defense against pecan scab because pecan varieties vary greatly in their susceptibility to pecan scab. Some varieties are resistant, but many grafted varieties are susceptible. Producers should keep in mind that most commercial varieties were at one time resistant to pecan scab and have now become susceptible because of genetic changes in fungus virulence. Some trees are resistant, but some are moderately susceptible. The grafted varieties 'Brewster,' 'Colby,' 'Giles,' 'Hirschi,' 'Neosho,' 'Osage,' 'Pawnee,' 'Peruque,' 'Ridgeway,' 'Shoal' and 'Stark's Hardy Giant' are susceptible to pecan scab. 'Hirschi' is highly susceptible to pecan scab and will be defoliated and suffer severe nut loss without protective fungicide sprays.

George S. Smith and Maureen H. O'DayDepartment of Entomology, University of Missouri-ColumbiaWilliam ReidKansas State University